Circuit Breaker Circuit Breaker Interrupting Capacity Interrupting Capacity and Shortand Short--Time Current RatingsTime Current Ratings
David D. Roybal, P.E.David D. Roybal, P.E.Fellow Application EngineerFellow Application EngineerEaton Electrical Eaton Electrical II CutlerCutler--HammerHammer
IEEE Industry Applications Society IEEE Industry Applications Society San Francisco Chapter San Francisco Chapter September 28, 2004September 28, 2004
September 28, 2004 IEEE San Francisco IAS2 of 31
Low Voltage Circuit BreakersLow Voltage Circuit BreakersLow Voltage Circuit Breakers
IEEE Definition:IEEE Definition:
A device designed to open and close a circuit A device designed to open and close a circuit by nonby non--automatic means, and to open the automatic means, and to open the circuit automatically on a predetermined circuit automatically on a predetermined overload of current without injury to itself overload of current without injury to itself when properly applied within its ratingswhen properly applied within its ratings
September 28, 2004 IEEE San Francisco IAS3 of 31
Molded Case Circuit BreakersMolded Case Circuit Breakers•• Tested in accordance with UL489Tested in accordance with UL489•• Open Air Test Open Air Test -- Rated @ 80%Rated @ 80%•• Over Toggle MechanismOver Toggle Mechanism•• Sealed Case Sealed Case -- Not MaintainableNot Maintainable•• Applied in Switchboards and PanelboardsApplied in Switchboards and Panelboards
Insulated Case Circuit BreakersInsulated Case Circuit Breakers•• Tested in accordance with UL489Tested in accordance with UL489•• Open Air Test Open Air Test -- Rated @ 80% or 100%Rated @ 80% or 100%•• 22--Step Stored Energy MechanismStep Stored Energy Mechanism•• Not Fully MaintainableNot Fully Maintainable•• Applied As Mains in Switchboards and MCCsApplied As Mains in Switchboards and MCCs
Power Circuit BreakersPower Circuit Breakers•• Tested in accordance with UL1066Tested in accordance with UL1066•• Tested in the Enclosure Tested in the Enclosure -- Rated @ 100%Rated @ 100%•• 22--Step Stored Energy MechanismStep Stored Energy Mechanism•• Fully MaintainableFully Maintainable•• MetalMetal--Enclosed DrawEnclosed Draw--out Switchgearout Switchgear
Low Voltage Circuit Breaker TypesLow Voltage Circuit Breaker TypesLow Voltage Circuit Breaker Types
September 28, 2004 IEEE San Francisco IAS4 of 31
Molded Case Circuit BreakersMolded Case Circuit BreakersInsulated Case Circuit BreakersInsulated Case Circuit Breakers
NEMA AB-1 – Molded Case Circuit Breakers and Molded Case Switches
UL489 – Molded-Case Circuit Breakers and Circuit-Breaker Enclosures
September 28, 2004 IEEE San Francisco IAS5 of 31
Low Voltage Power Circuit BreakersLow Voltage Power Circuit BreakersLow Voltage Power Circuit Breakers
ANSI C37.13 ANSI C37.13 –– IEEE Standard for LowIEEE Standard for Low--Voltage AC Power Circuit Breakers Used in Voltage AC Power Circuit Breakers Used in EnclosuresEnclosures
ANSI C37.16 ANSI C37.16 –– LowLow--Voltage Power Circuit Voltage Power Circuit Breakers and AC Power Circuit Protectors Breakers and AC Power Circuit Protectors --Preferred Ratings, Related Requirements, and Preferred Ratings, Related Requirements, and Application RecommendationsApplication Recommendations
UL1066UL1066 –– LowLow--Voltage AC and DC Power Voltage AC and DC Power Circuit Breakers Used in EnclosuresCircuit Breakers Used in Enclosures
September 28, 2004 IEEE San Francisco IAS6 of 31
The Maximum Short Circuit Current that the Circuit Breaker Can Safely Interrupt
at a Specific Voltage
The Maximum Short Circuit Current that The Maximum Short Circuit Current that the Circuit Breaker Can Safely Interrupt the Circuit Breaker Can Safely Interrupt
at a Specific Voltageat a Specific Voltage
Interrupting CapacityInterrupting CapacityInterrupting Capacity
• Expressed in rms symmetrical amperes
• Specified by current magnitude
September 28, 2004 IEEE San Francisco IAS7 of 31
The interrupting capacity for a circuit breaker provided with instantaneous trip elements is the maximum
rating of the device with no intentional delay
The interrupting capacity for a circuit breaker provided with instantaneous trip elements is the maximum
rating of the device with no intentional delay
Interrupting CapacityInterrupting CapacityInterrupting Capacity
The interrupting capacity for a circuit breaker providedwithout instantaneous trip elements is the maximum
rating of the device for the rated time interval
September 28, 2004 IEEE San Francisco IAS8 of 31
Short-time Current RatingShortShort--time Current Ratingtime Current Rating
Defines the Ability of the Device to Remain Closed for a Time Interval Under High
Fault Current Conditions
Defines the Ability of the Device to Remain Defines the Ability of the Device to Remain Closed for a Time Interval Under High Closed for a Time Interval Under High
Fault Current ConditionsFault Current Conditions
• Performance of a circuit breaker over a specific current range for a period of time
• Specified by current magnitude and time magnitude
• Allows system selectivity
September 28, 2004 IEEE San Francisco IAS9 of 31
Low Voltage Circuit Breaker TypesLow Voltage Circuit Breaker TypesLow Voltage Circuit Breaker Types
Molded Case Circuit Breaker (MCCB)Molded Case Circuit Breaker (MCCB)•• Tested in accordance with NEMA ABTested in accordance with NEMA AB--1 and UL4891 and UL489
Insulated Case Circuit Breaker (ICCB)Insulated Case Circuit Breaker (ICCB)•• Tested in accordance with NEMA ABTested in accordance with NEMA AB--1 and UL4891 and UL489
Power Circuit Breaker (PCB)Power Circuit Breaker (PCB)•• Tested in accordance with ANSI C37.13 and UL1066Tested in accordance with ANSI C37.13 and UL1066
September 28, 2004 IEEE San Francisco IAS10 of 31
Instantaneous Trip FunctionsInstantaneous Trip Functions
� All MCCBs and ICCBs are provided with instantaneous trip functions (no intentional delay)
� PCBs can be provided with or without instantaneous trip functions
September 28, 2004 IEEE San Francisco IAS11 of 31
ThermalThermal--magnetic trip elementsmagnetic trip elements•• Thermal element provided overload protectionThermal element provided overload protection•• Magnetic (instantaneous) trip elements provided Magnetic (instantaneous) trip elements provided
protection for highprotection for high--magnitude faultsmagnitude faults•• Available in both MCCBs and PCBsAvailable in both MCCBs and PCBs
Thermal elements only (for PCBs)Thermal elements only (for PCBs)•• Thermal element provided overload protectionThermal element provided overload protection•• 30 cycle short30 cycle short--time current ratingtime current rating•• No magnetic (instantaneous) trip elements providedNo magnetic (instantaneous) trip elements provided•• Interrupting rating was equal to the rating with Interrupting rating was equal to the rating with
magnetic elementsmagnetic elements•• Must be applied within their shortMust be applied within their short--time ratingtime rating
Vintage Low VoltageCircuit Breaker Trip Units
Vintage Low VoltageVintage Low VoltageCircuit Breaker Trip UnitsCircuit Breaker Trip Units
September 28, 2004 IEEE San Francisco IAS12 of 31
SolidSolid--state trip elements orstate trip elements ormicroprocessormicroprocessor--based trip elementsbased trip elements
•• LongLong--time pickup and delaytime pickup and delay
•• ShortShort--time pickup and delaytime pickup and delay
•• Instantaneous pickupInstantaneous pickup
•• GroundGround--fault pickup and delayfault pickup and delay
Newer Low VoltageCircuit Breaker Trip Units
Newer Low VoltageNewer Low VoltageCircuit Breaker Trip UnitsCircuit Breaker Trip Units
September 28, 2004 IEEE San Francisco IAS13 of 31
• Always provided in a molded case circuit breaker or an insulated case circuit breaker
• Sometimes called magnetic pick-up
• Instantaneous override
• Optional for power circuit breakers
Instantaneous Pick-upInstantaneous PickInstantaneous Pick--upup
September 28, 2004 IEEE San Francisco IAS14 of 31
Limited to the magnetic (instantaneous)pickup level of the device
••MCCBs MCCBs –– limited ratings which do not usually limited ratings which do not usually increase with higher interrupting capacityincrease with higher interrupting capacity
••ICCBs ICCBs –– some some extended ratingsextended ratings
••PCBs PCBs –– highest extended ratingshighest extended ratings
Short-time Current RatingShortShort--time Current Ratingtime Current Rating
September 28, 2004 IEEE San Francisco IAS15 of 31
Low Voltage Power Circuit Breakers Low Voltage Power Circuit Breakers --
•• Interrupting Rating:Interrupting Rating: Shall safely interrupt a rated fault Shall safely interrupt a rated fault current expressed in rms symmetrical amperes as measured current expressed in rms symmetrical amperes as measured 1/2 cycle after short circuit initiation1/2 cycle after short circuit initiation
•• ShortShort--Time Current Rating:Time Current Rating: Shall remain closed during a Shall remain closed during a short delay fault test of 30 cycle duration short delay fault test of 30 cycle duration -- a 15 second zeroa 15 second zerocurrent interval current interval -- followed by another 30 cycle fault durationfollowed by another 30 cycle fault duration
All ANSI C37 Tests are Performed at 15% Power Factor, or All ANSI C37 Tests are Performed at 15% Power Factor, or X/R Ratio of 6.6 or LessX/R Ratio of 6.6 or Less
ANSI C37 Test StandardANSI C37 Test StandardANSI C37 Test Standard
September 28, 2004 IEEE San Francisco IAS16 of 31
MCCBs and ICCBs MCCBs and ICCBs •• Highest interrupting capacityHighest interrupting capacity
•• Trips immediately for faults exceeding Trips immediately for faults exceeding instantaneous pickupinstantaneous pickup
•• Do not need to withstand high current for an Do not need to withstand high current for an extended time delayextended time delay
•• Economic advantagesEconomic advantages
PCBs PCBs •• High interrupting capacity availableHigh interrupting capacity available
•• Highest extended shortHighest extended short--time current ratingstime current ratings
Interrupting CapacityInterrupting CapacityInterrupting Capacity
September 28, 2004 IEEE San Francisco IAS17 of 31
MCCB Design Low IC High IC
Interrupting Capacity (kA @ 480 V)
35 100
Maximum Mag. Adjustment (kA)
4 4
Instantaneous Override (kA)
5 5
aThe minimum frame size for most ICCBs
and LVPCBs is 800 amperes.
Typical 400Typical 400--Ampere Frame Ampere Frame MoldedMolded--Case Circuit Case Circuit BreakersBreakersaa
September 28, 2004 IEEE San Francisco IAS18 of 31
Typical 800Typical 800--Ampere Frame Ampere Frame Circuit BreakersCircuit Breakers
Type of Device
MCCBs ICCBs LVPCBs
Low IC
High IC
Low IC
High IC
CL Low IC
(Internal
Inst. Trip)
High IC
(Internal
Inst. Trip)
CL (Inter
nal Inst. Trip)
Low IC (No Inst. Trip)
High IC (No Inst. Trip)
Interrupting Capacity (kA @ 480 V)
50 100 50 150 150 30 100 200 30 85
Instantaneous Override or Max. Short-time Current Rating (kA)
6 – 9 6 – 9 25 25 30 30 85 30 30 85
Short-time Delay(cycles)
18 18 30 30 30 30 30 30 30 30
September 28, 2004 IEEE San Francisco IAS19 of 31
Typical 1600Typical 1600--Ampere Frame Ampere Frame Circuit BreakersCircuit Breakers
Type of Device
MCCBs ICCBs LVPCBs
Low IC
High IC
Low IC
High IC
CL Low IC
(Internal
Inst. Trip)
High IC
(Internal
Inst. Trip)
CL (Inter
nal Inst. Trip)
Low IC (No Inst. Trip)
High IC (No Inst. Trip)
Interrupting Capacity (kA @ 480 V)
65 100 65 150 150 50 100 200 42 85
Instantaneous Override or Max. Short-time Current Rating (kA)
17 17 35 51 30 42 85 30 42 85
Short-time Delay(cycles)
18 18 30 30 30 30 30 30 30 30
September 28, 2004 IEEE San Francisco IAS20 of 31
Externally adjustable designs• Typical adjustment range of 5 – 10 times the
frame continuous ampere rating• Inhibits the use of a higher instantaneous
pickup that may be available
Internally fixed designs• MCCBs set at about 13 times the frame rating• ICCB setting may be higher• May be deleted for PCBs
Both externally adjustable and internally fixed instantaneous trip elements inhibit the use of short-time
current ratings
Instantaneous SettingsAdjustable versus FixedInstantaneous SettingsInstantaneous SettingsAdjustable versus FixedAdjustable versus Fixed
September 28, 2004 IEEE San Francisco IAS21 of 31
X/R Ratio - Impedance DiagramX/R Ratio X/R Ratio -- Impedance DiagramImpedance Diagram
September 28, 2004 IEEE San Francisco IAS22 of 31
Current in Per Unit20
15
10
5
0
-5
-10 Time in Cycles
0 1 2 3 4
X/R Ratio Effect on SymmetryX/R Ratio Effect on SymmetryX/R Ratio Effect on Symmetry
X/R = 0, PF = 1.0 (symmetry) X/R = 6.6, PF = 0.15 (asymmetry)X/R = 0, PF = 1.0 (symmetry) X/R = 6.6, PF = 0.15 (asymmetry)
September 28, 2004 IEEE San Francisco IAS23 of 31
Asymmetrical Current WaveAsymmetrical Current WaveAsymmetrical Current Wave
September 28, 2004 IEEE San Francisco IAS24 of 31
Circuit Breaker Circuit Breaker Asymmetrical RatingsAsymmetrical Ratings
Type of Device MCCB ICCB LVPCB
Test PF (%) 20 20 15
X/R 4.9 4.9 6.6
Peak Mult. Factor
2.2 2.2 2.3
September 28, 2004 IEEE San Francisco IAS25 of 31
Peak Multiplication Factor
X/R Ratio - Application DataX/R Ratio X/R Ratio -- Application DataApplication Data
September 28, 2004 IEEE San Francisco IAS26 of 31
• Protection and coordination are often competing objectives
• Selective tripping is necessary for continuity of service
• Coordination is achieved through the use of short-time current ratings
• Selective coordination may be sacrificed with:Series ratings – instantaneous tripsCurrent limiting circuit breakers
Equipment Protection ConsiderationsEquipment Protection ConsiderationsEquipment Protection Considerations
September 28, 2004 IEEE San Francisco IAS27 of 31
Time-Current CoordinationSystem One-line Diagram
TimeTime--Current CoordinationCurrent CoordinationSystem OneSystem One--line Diagramline Diagram
2500 kVA Tx5.75% Z
4000 A
800 A
250 A
Isc = 56.6 kA
September 28, 2004 IEEE San Francisco IAS28 of 31
TimeTime--Current Curves for a Current Curves for a NonNon--Coordinated SystemCoordinated System
September 28, 2004 IEEE San Francisco IAS29 of 31
TimeTime--Current Curves for a Current Curves for a Coordinated SystemCoordinated System
September 28, 2004 IEEE San Francisco IAS30 of 31
Summary
�� All circuit breakers provide overcurrent protectionAll circuit breakers provide overcurrent protection
�� Performance based on design standards and Performance based on design standards and featuresfeatures
�� Interrupting capacity, shortInterrupting capacity, short--time current ratings, time current ratings, test power factor (X/R) determine ability to provide test power factor (X/R) determine ability to provide system protection, coordination, and selectivitysystem protection, coordination, and selectivity
September 28, 2004 IEEE San Francisco IAS31 of 31
ConclusionsConclusionsConclusions• MCCBs and ICCBs
• Excellent interrupting ratings but limited short-time current ratings
• For improved selectivity choose a MCCB or an ICCB with a fixed internal instantaneous trip unit rather than an externally adjustable trip unit
• LVPCBs• Available without instantaneous trip elements and
with high short-time current ratings• Some lower short-time current rating models are
being introduced
High interrupting capacity does not necessarily mean high short time current rating